I. Field of the Invention
This invention relates to aqueous compositions and processes for using these in cleaning aluminum surfaces without causing significant discoloring or tarnishing of the metal. More specifically, the invention concerns the use of small amounts of sodium metasilicate alongside either alkali metal carbonates or orthophosphates in cleaning formulations to substantially reduce or altogether prevent alkali attack on aluminum.
II. The Prior Art
Highly alkaline solutions have proved very effective for the cleaning of soft metals such as aluminum. These solutions easily remove baked-on food, oleo resinous films, fatty soils, oxidized hydrocarbons, waxy deposits, carbonaceous soils and similar encrustations which are difficult to remove with less highly alkaline compositions. Unfortunately, alkalis readily corrode and dissolve soft metals. Metal discoloration, tarnishment and even pitting occur under highly basic conditions.
One response to the problem has been replacement of strong with neutral or mildly alkaline solutions that depend primarily on detergent action. For the more tenaciously held soils, the detergent action of surfactants have proved ineffective. Only light duty cleaning operations are practical for surfactants.
Sodium silicate has been widely used in passivating aluminum surfaces. However, sodium silicate cleaners suffer from several limitations. The most serious is the restriction on level of alkalinity. Therefore, the high alkalinity necessary for the removal of many soils cannot be used. Furthermore, long soaking periods or mechanical action is necessary to accomplish the release of soil.
Barium and mercury salts have been reported to potentiate the corrosive effects of the alkaline environment. In U.S. Pat. No. 2,303,398, mercuric chloride reduced the corrosion of a soft metal (tin) over that of an aqueous solution containing sodium metasilicate alone, trisodium orthophosphate alone or combinations of metasilicate and orthophosphate. Aluminum was suggested as having alkaline corrosion properties similar to that of tin. Another patent, U.S. Pat. No. 3,655,582, discloses that mixtures of barium salts with sodium metasilicate can control aqueous sodium or potassium hydroxide corrosion of aluminum.
Smectite and attapulgite clays have been described in U.S. Pat. Nos. 4,116,849 and 4,116,851 as corrosion protection agents alongside sodium silicates in aqueous alkaline hypohalite cleaners. These cleaners were directed towards pre-treating kitchen housewares, especially pots, pans, dishes, etc., which were coated with hard-to-remove food soils.
Those anti-corrosion additives of the prior art suffer a number of shortcomings. Some are ecologically toxic; others expensive. Still others are simply not effective enough under highly alkaline conditions. Thus, there continues to be a need for an aluminum surface cleaner which exhibits the efficiency of highly alkaline compositions without the attendant shortcomings.
None of the foregoing art has suggested the synergistic relationship between sodium metasilicate and either alkali metal carbonates or orthophosphates. Neither have the criticality of concentration ratios and pH ranges been previously disclosed.
The object of the present invention is to provide a simple but effective means for cleaning aluminum surfaces.